Epoxy Adhesives, Part 2

In the last article the discussion on formulating epoxy adhesives started with the epoxy resins and curing agents. It will continue with a review of modifiers. Epoxy resins can accommodate significant addition of fillers, etc., without adversely affecting desired adhesion properties. This makes them versatile for formulating to meet the specific requirements of a wide range of applications.

To reduce the viscosity of the epoxy adhesive, and potentially allow for greater filler loading, diluents may be added to the formulation. There are reactive diluents, which become part of the polymer matrix when the adhesive is cured, and nonreactive diluents, or plasticizers, which remain separate. The concern with plasticizers is that, while they may be lower in cost, they may also reduce cured adhesive properties more than the reactive diluents. In addition, similar to plasticizer use in other plastics, they may migrate out of the cured adhesive. If this occurs at the bondline, there could be loss of adhesion.

To increase the toughness, characterized by resistance to peel and greater impact strength, a flexibilizer may be added. These are elastomeric modifiers that react and become part of the polymer matrix. At lower loading levels of the flexibilizer there is little loss of modulus or hot strength. At higher loading levels, there is some loss of room temperature and hot strength and stiffness, while achieving resistance to thermal shock and increases in tensile elongation. For applications with significant thermal cycling, it may be more important to have increased resistance to the thermal cycling with some reduced, but still adequate, adhesion strength.

Fillers perform an important function in reducing the cost of epoxy adhesives while fine tuning the adhesive to meet processing and application requirements. Alumina or metallic fillers, such as silver, will provide improved electrical conductivity. Add colloidal (fumed) silica for improved thixotropy. Some fibrous talcs (hydrous magnesium silicate) could also be used for improved handling and thixotropy, but there may be a health concern in the manufacture of the adhesive that would need to be managed. Platy talcs (also hydrous magnesium silicate) do not have the same health concerns and can be used instead to reduce cost and increase viscosity, but they do not achieve the same level of thixotropy as the fibrous talcs. Improved impact strength can be achieved with the addition of mica, silica, or glass - in powdered or flaked form. For improved thermal conductivity, add calcium carbonate or calcium silicate or powdered metal, such as aluminum or copper. Aluminum, silica, and molybdenum disulfide are some of the materials that could be added for improved machinability of the cured adhesive.

The various fillers may also affect cure rate, the temperature required for cure, and/or the temperature reached in exothermic reactions. One specialized cure process, induction curing, works by manipulating magnetic fields to heat a localized area. For most bonds the substrate contains iron which responds to the magnetic field, heats, and cures the adhesive to the desired state – either fully or just to a “gelled state” to achieve handling strength. For substrates such as FRP (fiber reinforced plastic), the adhesive formulation will include an additive, such as an iron powder, that will respond to the magnetic field, which in turn heats and cures the adhesive.

In the next article in this series, additional manufacturing and processing considerations will be discussed.

Note: references will be included at the conclusion of this series on epoxy adhesives.